1. Field of the Invention
This invention relates to a method for insulating superconductors in a magnet winding, in which sizing and/or binders which contain organic substances and are deposited on heat resistant insulators, are removed prior to an in-situ anneal of intermediate conductor products provided for forming the superconductive properties of the conductors.
2. Discussion of the Prior Art
Superconductive intermetallic compounds of the type A.sub.3 B having an A15 crystal structue, such as Nb.sub.3 Sn or V.sub.3 Ga, have good properties of superconduction and are distinguished by high critical values. Conductors using these materials are therefore especially well suited for use in supercondcuting magnet coils for generating strong magnetic fields. In addition to these binary compounds, ternary compounds, such as niobium aluminum germanium Nb.sub.3 Al.sub.0.8 Ge.sub.0.2, are of particular interest for use as conductors in such magnets.
However, these intermetallic compounds are generally very brittle, so that their manufacture in a form suitable for magnet coils presents difficulties. Special processes have, therefore, been developed, by which superconductors having the A15 crystal structure can be fabricated in the form of long wires or ribbons. In these processes, which make possible, in particular, the fabrication of so-called multicore conductors, a first component, which is a dectile element in wire form of the intermetallic compound to be manufactured, is generally surrounded by cladding which consists of a ductile carrier metal and an alloy containing the other elements of the compound. For instance, a wire of niobium or vanadium is surrounded with cladding of a copper-tin bronze or a copper-gallium bronze. A multiplicity of such wires can also be embedded in a matrix of the alloy. The assembly of these two componets thus provided is then subjected to a cross-section reducing treatment and a long, wire-like structure is obtained, such as is needed for coils, without the occurrence of reactions which would embrittle the conductor. After the cross-section is reduced, the intermediate super-conductor product, consisting of one or several wire cores and the surrounding matrix material, is then subjected to an annealing treatment to form the desired superconductive compound, having an A15 crystal structure, by a reaction of the core material with the other element of the compound which is contained in the surrounding matrix. The element contained in the matrix thus is diffused into the core material, forming the compound (See British Patent No. 1,280,583).
Superconducting magnet coils using such superconductors are generally made by two different methods. In the first method, which is also known as the "react first-then wind" method, the intermediate conductor product of the superconductor to be manufactured is wound on a temporary coil form and is then subjected to the annealing treatment required for forming the desired superconductive compound. Subsequently, the superconductor made in this manner is unwound from the temporary coil form and can be processed further. The danger xists, especially when winding magnet windings, that the brittle intermetallic compounds of the conductors may be damaged, due to excessive deformation of the conductor, and that their superconduction properties would be impaired accordingly.
This danger does not exist with the second method of manufacturing the superconductive compound from the intermedite conductor product. In this method, which is called "wind-and-react" technique, the coil form of the magnet to be provided with the winding is first wound with not-yet-fully-reacted intermediate conductor product, and then the entire magnet, wound in this manner, is subjected to the diffusion anneal. This anneal is also called "in-situ" anneal. With this procedure, all difficulties of processing a brittle conductor material are avoided. It is also possible, in this manner, to fabricate coils having small inside diameters using relatively heavy conductors. However, with this method, all materials must withstand the high temperatures required for the diffusion anneal, which, for instance, in the case of niobium-tin, may be in the range of 700.degree. C., for several hours.
Because of these requirements, almost the only insulating materials which can serve for insulating the turns and layers of the magnet winding are ceramics, glass, or quartz, in the form of filaments, fabrics or nonwoven fabrics. So that these, generally very brittle, insulating materials can be handled at all, so-called sizings and/or special binders are applied to them during their manufacture, to increase their notch impact strength and cohesion.
Sizing for fibers of the insulating materials mentioned may consist of an adhesion or film-forming agent, a lubricant and a wetting agent. Optionally, adhesion additives can be provided. These sizings contain, for instance, starch, dextrin or polyvinyl acetate (PVAC) as the adhesion and film-forming agent, and, as a rule, vegatable fats or oils as lubricants, and surface-active substances as wetting agents. Binders for fabrics of the insulating materials mentioned generally contain organic substances of the varnish or wax type. Such binders are, for instance, polyurethane or polyvinylbutyral.
The intermediate conductor products are generally braided or wrapped with glass or quartz filaments. In order to increase the resistance of such insulation against mechanical stresses, the insulation so prepared is generally also impregnated with a binder of the varnish or wax type. Even so, simple coverings do not provide sufficient security against shorts between turns. Therefore, multiple coverings or braids are provided which, however, result in a substantial increase in thickness and therefore, especially in the case of thin conductors, result in a corresponding decrease of the turns density in the winding. Because of the higher induced voltage between layer of a winding and for reasons of winding technique, fiberglass layer insulation is usually inserted, in addition.
In order to drive out the organic components in the winding, which stem from the sizing and the binders, the magnet windings are generally subjected to a purification anneal at temperatures of between 240.degree. C. and 400.degree. C., for instance, prior to the diffusion anneal of the intermediate conductor products. Carried out in a vacuum or in air, loss of more volatile components of the interim conductor product, such as tin, can occur here which decreases the current-carrying capacity of the subsequently annealed superconductor. In addition, oxides form on the matrix material, which diffuse at higher temperatures, for instance, above 700.degree. C., into the glass material, and lead to complete embrittlement as well as to a decrease of the melting point of the glass. There is also the danger of a mechanical destruction of the insulation. If, on the other hand, the purification anneal is carried out in a protective gas such as argon, then the organic substances are only partially driven out of the winding; the rest is decomposed in the subsequent diffusion anneal to form graphite. This impairs the insulation and can result in short circuits in the winding.
It is therefore an object of the present invention to provide a method for insulating superconductors of a magnet winding to be annealed in situ, in which these dangers do not occur.